1. Field of the Invention
This invention relates to a method and apparatus for correcting errors in encoded uncorrected data in a storage device, and more particularly to a method and apparatus using a unique implementation involving look-up tables for decoding syndromes of error indicative of at least two errors in said data.
2. Description of Background Art
The following references disclose basic theories and various methods and arrangements for correcting errors in storage devices: W. W. Peterson, Error-Correcting Codes, M.I.T. Press, 1961; R. C. Bose and D. K. Ray-Chaudhuri, "On a class of error-correcting binary group codes", Inf. Control 3, pp. 68-69, 1960; I. S. Reed and G. Solomon, "Polynominal codes over certain finite fields", J. Soc. Indust. Appl. Math 8, pp. 300-304, 1960; and R. T. Chien, "Cyclic decoding procedures for Bose-Chaudhuri-Hocquenghem codes", IEEE Trans. Inf. Theory, Vol. IT10, pp. 357-363, 1964.
In the copending application U.S. Ser. No. 781,449, filed Sept. 27, 1985 (now U.S. Pat. No. 4,706,250, issued Nov. 10, 1987), assigned to the assignee of the present invention, there is disclosed a two-level error correction code structure in an improved multibyte error-correcting subsystem. Data is formatted on a disk track into a number of subblocks, each within a respective block. There are also two sets of three subblock check bytes C.sub.1, C.sub.2 and C.sub.3. One set is associated with the even phase and the other with the odd phase, thus providing interleaved codewords. With this arrangement, the first level of correction (of subblock errors) is done on-the-fly at the storage device after being delayed one subblock, and the data is sent to the storage director for correction of any second level (block) errors. This on-the-fly correction is suitable only for systems operating in an asynchronous environment. The second (block) level correction is carried out using one additional check byte C.sub.0 for each phase at the end of the block.
The aforementioned copending application discloses hardware in a disk storage device that receives uncorrected data in real time and generates three first-level syndrome bytes and one second-level syndrome byte (corresponding to each phase). The first-level syndromes are decoded at the device into error pattern and error location information that is transmitted to a storage director.
This aforementioned application describes how the system functions if there are no errors in any subblock, and how errors are corrected if there is not more than one error per subblock and if there is more than one error per subblock. More specifically, syndromes S.sub.1, S.sub.2, S.sub.3 corresponding to each phase associated with a particular subblock codeword are held in a local memory. They are retained for further processing at the block level if that particular subblock's syndromes were not all zeros and no nonzero error pattern was generated by a first level decoder. The local memory also retains the identification of an uncorrected subblock as subblock identifier "f". At the end of the block, a second level syndrome, S.sub.0, from a second level syndrome generator and the first level syndromes S.sub.1, S.sub.2, S.sub.3 for subblock f from the local memory are processed by a second level decoder to correct two errors in subblock f.
There is a need for a less expensive, yet efficient, method and apparatus which uses unique look-up tables implemented in firmware or software to decode the syndromes for two-byte errors in a record of a predetermined number of bytes, in which each byte contains a desired preselected number of bits; e.g., two-byte errors in any one subblock of a two-level code format similar to that described in said copending application.